There is an increasing demand for fired pellets as a raw material for blast furnace or direct-reduction ironmaking. Fired pellets are usually manufactured as follows: adding a powdery flux to raw materials comprising an iron ore fine to form a mixture, forming the mixture into green pellets, and firing the thus formed green pellets into fired pellets. Many studies have been made to improve the quality of the fired pellets. For example, a method for continuously manufacturing fired pellets is disclosed in Japanese patent provisional publication No. 58-9,936 dated Jan. 20, 1983, which comprises the steps of:
using a raw material comprising an iron ore fine having a particle size of up to 5 mm; adding a powdery flux, a powdery solid fuel and water to said raw material to form a mixture; forming said mixture into green pellets having a particle size of from 10 to 20 mm; using an endless travelling grate type firing furnace comprising a first drying zone, a second drying zone following said first drying zone, an ignition zone following said second drying zone, a firing zone following said ignition zone, and an endless travelling grate passing sequentially through said zones; feeding said green pellets onto said endless travelling grate at the inlet side thereof; causing said green pellets on said endless travelling grate to travel sequentially through said first drying zone, said second drying zone, said ignition zone and said firing zone; blowing a first drying gas at a temperature of from 150.degree. to 350.degree. C. into said first drying zone from below upwardly to conduct a primary drying of said green pellets in said zone; blowing a second drying gas at a temperature of from 150.degree. to 350.degree. C. into said second drying zone from above downwardly to conduct a secondary drying of said green pellets in said zone; igniting said powdery solid fuel contained in said green pellets in said ignition zone; and downwardly sucking a combustion exhaust gas produced by combustion of said powdery solid fuel contained in said green pellets through said green pellets in said firing zone to heat said green pellets in said zone to a firing temperature, thereby firing said green pellets into fired pellets (hereinafter referred to as the "prior art 1").
The prior art 1 has the following problems:
(1) The raw material comprises an iron ore fine having a particle size of up to 5 mm, and the particle size distribution of the iron ore fine is not defined. Therefore, when an iron ore fine having a particle size of from over 0.5 mm up to 5 mm is present in the raw material in an amount of over 70 wt. %, the iron ore fine is hard to combine together when forming it into green pellets. As a result, the green pellets tend to easily disintegrate during transferring and firing thereof.
(2) When an iron ore fine having a particle size of up to 0.044 mm is present in the raw material in an amount of over 80 wt. %, the green pellets would have a higher bulk density. As a result, steam-bursting causes the green pellets to disintegrate when drying and firing the green pellets. In order to prevent disintegration of the green pellets caused by steam-bursting, in the prior art 1, the first drying zone and the second drying zone are provided in the upstream of the ignition zone of the endless travelling grate type firing furnace. In the first drying zone, the primary drying of the green pellets is conducted by means of the first drying gas blown upwardly from below, and in the second drying zone, the green pellets in this zone are subjected to the secondary drying by means of the second drying gas blown downwardly from above. However, since the first drying zone and the second drying zone are provided in the endless travelling grate type firing furnace as described above, a larger area in this firing furnace is required for drying the green pellets, with a decreased production efficiency of the green pellets and increased equipment and running costs.
(3) The green pellets have a particle size of from 10 to 20 mm. When firing the green pellets having such a large particle size into fired pellets, a difference in temperature is produced between the surface and the center portion of the green pellets, thus causing the green pellets to easily disintegrate.
(4) The fired pellets have a particle size of from about 10 to 20 mm just as the green pellets. When the fired pellets having such a large particle size are charged into a blast furnace, it takes much time for a reducing gas to penetrate into the center portion of the fired pellets. As a result, reducibility of the fired pellets in the blast furnace degrades, and the cores of the fired pellets remaining unreduced cause degradation of high-temperature property under load of the fired pellets.
Fired pellets with a limited particle size distribution of an iron ore fine are disclosed in Japanese patent publication No. 55-27,607 dated July 22, 1980, wherein:
a raw material for fired pellets comprises a first iron ore fine of under 70 wt. % and a second iron ore fine of at least 30 wt. %; said first iron ore fine contains at least 70 wt. % iron ore fine having a particle size of up to 0.044 mm, and has a basicity of at least 1.0; and said second iron ore fine has a particle size of from at least 0.177 mm up to 1.0 mm (hereinafter referred to as the "prior art 2").
The above-mentioned prior art 2 has the following problems:
(1) Since the second iron ore fine has a small particle size of from at least 0.177 mm up to 1.0 mm, the number of macro-pores in the fired pellets decreases, thus causing reducibility of the fired pellets charged into a blast furnace to degrade, and the cores of the fired pellets remaining unreduced cause degradation of high-temperature property under load of the fired pellets.
(2) As described above, the particle size of the second iron ore fine is limited within the range of from at least 0.177 mm up to 1.0 mm. In order to limit the particle size of the second iron ore fine to such a low level, it is necessary to finely crush the iron ore and conduct screening many times. As a result, crushing and screening of the iron ore require considerable expenses, resulting in a higher manufacturing cost.
A method for manufacturing lumpy fired pellets in which a plurality of fired pellets are combined into a lump, is disclosed in Japanese patent publication No. 58-53,697 dated Nov. 30, 1983, which comprises the steps of:
adding a powdery flux and water to a raw material comprising an iron ore fine to form a mixture; forming said mixture into green pellets having a prescribed particle size; covering the surfaces of said green pellets with a mixture of a powdery solid fuel and a powdery silica; firing said green pellets into fired pellets in an endless travelling grate type firing furnace; whereby, in said firing step, fayalite is formed on the surfaces of said fired pellets and said fayalite combines a plurality of said fired pellets into a lump (hereinafter referred to as the "prior art 3").
When the lumpy fired pellets manufactured according to the prior art 3, in which a plurality of fired pellets are combined into a lump, are charged into a blast furnace, the lumpy fired pellets have an advantage of not impairing smooth passage of a reducing gas because the lumpy fired pellets never flow preferentially into the center portion of the blast furnace and gaps are produced between the lumpy fired pellets. However, the prior art 3 has the following problem: the fired pellets manufactured according to the prior art 3 are combined into a lump by means of fayalite having a low reducibility. The lumpy fired pellets have therefore a low reducibility.
Under such circumstances, there is a strong demand for the development of a method for economically and continuously manufacturing fired pellets at a high yield, which have a high strength and an excellent reducibility, and do not impair smooth passage of a reducing gas in the blast furnace, and wherein green pellets do not disintegrate during transferring and firing thereof. However such a method has not as yet been proposed.